1. Field of the Invention
The present invention relates to a two-station type color image forming apparatus applicable to, e.g., a copier, a printer or a facsimile apparatus and more particularly to a method of assembling a two-station type color image forming apparatus and an image carrier unit.
2. Description of the Background Art
In an image forming apparatus, a photoconductive drum or image carrier and at least one of a developing device, a charger and a cleaning device may be constructed into a single process cartridge removable from the body of the apparatus, as taught in Japanese Patent Laid-Open Publication No. 2000-72733 by way of example. The process cartridge allows the user of the apparatus to easily perform replacement and maintenance without relying on a service person. However, in the case where the process cartridge includes the drum and image forming means, the process cartridge must be bodily replaced when only the drum or only part of the image forming means should be replaced.
In the process cartridge, the drum and a cleaning case rotatably supporting the drum are constructed integrally with each other. Also, process means for forming an image on the drum are mounted on the cleaning case. The process means include a charge roller or charger for uniformly charging the drum and a cleaning blade and a cleaning roller for removing toner left on the drum after the transfer of a toner image to a sheet or recording medium. Such process means are arranged around the drum.
The process cartridge is removably mounted to the apparatus body and is replaceable when the life of the drum ends or when the cleaning case is filled up with waste toner.
On the other hand, Japanese Patent Laid-Open Publication Nos. 10-177286 and 11-295952, for example, each disclose a two-station type recording system in which a developing device, a writing device and drive means are mounted to an apparatus body via common mount members at each of two image stations and accurately positioned relative to each other. In this type of recording system, the developing device defines a reference position for all of image forming process devices to be mounted.
The drum or drum unit is not mounted to the apparatus body, but is mounted to the developing device. More specifically, because the drum or the drum unit is positioned relative to only the developing device or developing unit, the former is subsidiary to the latter. Further, the drum or the drum unit is removable from the developing device, which is, in turn, removable from the apparatus body. In addition, the drum, charging means and cleaning means are constructed integrally with each other.
There is an increasing demand for a printer, copier or similar image forming apparatus having advanced configurations that, in turn, make loads on an image forming device heavier during image formation. A series of studies and experiments showed that the advanced configurations desired on the market tend to increase loads on, among various image forming means, the drum, as will be described hereinafter.
First, it is necessary to reduce the size of an image forming device in order to meet the increasing demand for small-size office automation equipment. However, if the size or diameter of the drum is reduced, then the drum is more exhausted for a single print under given conditions. For example, if the diameter of the drum is reduced from 130 mm to 40 mm, then the drum must rotate three times more for a given image size. It follows that the drum suffers from various kinds of exhaustion including electric exhaustion ascribable to, e.g., the discharge of a charger and mechanical exhaustion ascribable to a blade included in a cleaning section three times more.
While a certain degree of size reduction has already been implemented with, e.g., a developing device, the drum has not been reduced in size like the developing device. Reducing the size of the drum, however, increases loads on the drum and thereby reduces the life of the drum.
Second, the ratio of photographic images and graphic documents to the entire documents to be dealt with by users is increasing today, so that image quality as high as one achievable with silver halide type of photography is desired. While such high image quality may typically be implemented by high resolution, high resolution is not attainable with electrophotography unless a photoconductive layer formed on the drum is made thin. For example, in a photoconductive layer chargeable to negative polarity, a charge carrier generated in a CGL (Charge Carrier Generation Layer) by exposure is propagated to the surface of the photoconductive layer via a CTL (Charge Carrier Transport Layer), forming a latent image on the photoconductive layer. At this instant, if the CTL is thick, then the carrier must be propagated over a long distance and therefore separates due to electric repulsion. This prevents a latent image faithful to image data from being formed on the photoconductive layer, i.e., prevents dots from being faithfully formed at expected positions. This problem arises not only when resolution is increased from 600 dpi (dots per inch) to 1,200 dpi, but also when higher image quality is desired with resolution being maintained at, e.g., 600 dpi.
To obviate the degradation of image quality mentioned above, it is necessary to reduce the thickness of the photoconductive layer for thereby reducing the distance over which the charge carrier is propagated. However, the photoconductive layer is shaved off by the cleaning blade or otherwise exhausted every time an image is formed thereon. The life of the photoconductive layer therefore becomes shorter as its thickness decreases.
Third, a color image, which is increasing on the market because it renders information easy to understand, differs from a black-and-white or text image in that in many cases a photographic image or a graphic image occupies the major area of a sheet. In addition, the background of a color image is often a solid image. As a result, the image forming area for a single image formation increases and aggravates the exhaustion of the image forming device including the drum.
An image forming apparatus of the type including a revolver made up of a plurality of developing sections is also extensively used on the market because it needs a minimum of parts and can form a color image at relatively low cost. However, this type of image forming apparatus causes the developing sections to form respective images on a photoconductive drum, so that the drum is exhausted several times more than the individual developing section. In this manner, the current trend to color image formation reduces the life of the drum also.
The demands for smaller configuration, higher image quality and color image formation described above will make the life of the drum shorter in the future relative to the life of the other image forming devices. More specifically, the life of the drum tends to decrease relatively because researches and experiments are under way for enhancing not only the durability and life of the drum, but also those of the other developing devices. This brings about unbalance between the drum and the other image forming means mounted on the process cartridge.
More specifically, the problem with the process cartridge heretofore pointed out is that the process cartridge must be bodily replaced when the life of image forming means shorter than the lives of the image forming means ends. This problem is becoming more serious with the decreasing life of the drum, i.e., the image forming means longer in life than the drum must be replaced together with the drum whose life is shortest. Discarding or recycling the image forming means still usable would aggravate economic loads on the user, waste of time and labor necessary for collection, and have adverse influence on the environment.
To solve the above problems, Japanese Patent Laid-Open Publication No. 2000-298315 proposes an image forming apparatus, an image carrier unit and so forth configured such that, among various structural elements constituting image forming means, a structural element whose life is shortest is replaced before the others. Although a developing device included in this image forming apparatus can be replaced by the user, gears and other drive members associated with the developing device are bare and apt to smear or hurt the user's hand and bring about trouble in the drive members. Moreover, the user cannot replace the revolver type developing device that is rotatable for switching color.